bot events: Prevent duplicate add-bot notifications.

We don't need `do_create_user` to send a partial
event here for bots.  The only caller to `do_create_user`
that actually creates bots (apart from some tests that
just need data setup) is `add_bot_backend`, which
sends the more complete event including bot "extras"
like service info.

The modified event tests show the simplification
here (2 events instead of 3).

Also, the bot tests now use tuple unpacking, which
will force a ValueError if we duplicate events
again.

zerver/lib/actions.py

@@ -523,10 +523,10 @@ def do_create_user(email: str, password: Optional[str], realm: Realm, full_name:
     if settings.BILLING_ENABLED:
         update_license_ledger_if_needed(user_profile.realm, event_time)
 
+    # Note that for bots, the caller will send an additional event
+    # with bot-specific info like services.
     notify_created_user(user_profile)
-    if bot_type:
-        notify_created_bot(user_profile)
-    else:
+    if bot_type is None:
         process_new_human_user(user_profile, prereg_user=prereg_user,
                                newsletter_data=newsletter_data,
                                default_stream_groups=default_stream_groups,